Signal transmission system



'ceiving end of Patented Oct. 23, 1945 2,387,652 SIGNAL TRANSMISSION SYSTEM Alton c. Diekieson,

Mountain Lakes, N. J., as-` signor to Bell Telephone Laboratories, Incorporated, New York, York N. Y., a kcorporatorrof New Application October 1, 1942, Serial No. 460,389

` 7 Claims. (Cl. 178-44) This invention relates to signal transmission systems and particularly to volume control circuits for such systems.

As has been set forth in earlier patents, it is often desirable in connection with the transmission of signals having an extremely wide range of volumes over a transmission system in which the volume range of currents which can be satisfactorily transmitted is limited, to compress the signal energy volume range at the transmitting end of the 'system and to expand the received signals to their original volume range at the rethe system. The arrangements utilized for these purposes, commonly known as compressor and expander respectively,yhave generally involved relatively expensive elements and their use has been confined mostly tolso-called high quality transmission systems.

An object of the present invention is to simplify and reduce the cost of compressors and expanders and thereby render their use feasible in connection with relatively inexpansive transmis! sion systems.

Arrangements have previously been proposed l see, for example, Vario-losser circuits, by W. R. Bennett and S. Doba, published in Electrical Engineering (Transactions Section), vol. 60, pages 17-22, January 1941) in accordance with which compression of a wave in a` ratio of 2:1 is attained by connection of two identical variolossers in series. The same biasing 7current is suppliedto each vario-losser from a marginal rectifier whereby the output from thesecond losser is held constant whenever the input to the iirst exceeds the so-called critical value of the marginal rectifier. The compressed signal outp-ut is taken oil" at a point betweenA the output of the first losser and the input to the second losser. As each losser has the sam'e loss and as the output from the second is held constant it follows that the output from the first losser will increase Only decibels when the input increases by X decibels, the desired compression ratio of 2:1. The above article also points out that a marginal rectifier l or detector) may be defined as a device which delivers no output when the input is below a cervario-losser (to wit: the varistors when the varistor Atype, of vario-losser is utilized) need have no specific current-voltage `characteristics or any other property other than that of 'resistance variable with bias in order to obtain speci- \\ed ratios of compression.

' In accordance with a feature of the present invention as incorporated in one embodiment thereof a result similar to that arrangement referred to above similar advantages is attained one vario-losser by the novel expedient of passing certain currents distinguishable one from the other on a frequency basis through the single vario-losser twice. 'I'his novel arrangement entirely eliminates problems attendant upon the provision of two identical vario-lossers and maand one offering terially simplifies the provision of volume control circuits. x

The general arrangement of the present invention, like that of the article referred to above. is applicable to expanders as well as to compressors.

Other features of the invention will be apparent from the following detailed description of several embodiments thereof when considered in connection with the annexed drawings in which:

Fig. 1 illustrates a 2:1 compressor circuit;

Fig. 1a is a curve illustrating marginal rectier characteristics;

Fig. 2 illustrates a 2:1 expander circuit utilizing a feedback amplifier as one element;

Fig. 3 illustrates another form of 2:1 compressor circuit;

Fig. 4 illustrates still another form of 2:1-compresser circuit; and

Fig. 5 illustrates a 2:1 expander circuit which 'includes two vario-lossers.

Referring now to Fig. 1 a portion of a speech transmission line Il is schematically illustrated, the illustrated portion including vario-losser l2, double-balanced modulator I3 and the bandpass filter I4. These elements are well kno-wn and various forms of each have been described heretofore in technical papers and the patented art. Vario-losser l2, for example, may be of the varistor type as illustrated in the drawings and as' described in detail in the Bennett-Deba article referred to above or may comprise a balanced pair of vacuum tubes whose gain is changed bylvaryingthe grid bias. (See trolling amplitude characteristics of telephone signals, by A. C. Norwine, Bell System Technical Journal, vol. 17, No. 4, page 539.) Moduachieved by the by use of only Devices for conlator I3 may be one of the double-balanced types described in Copper oxide modulators in carrier telephone systems, by R. S. Carruthers, Bell System Technical Journal, April 1939, page 315. Filter I4 in this instance is designed to pass to the output end of the line only a small band of frequencies which band includes C+V. In other cases the lter may be designed to pass C-V.

Assuming now that voice frequency input current V be applied to the input end of line I I, it is passed through vario-losser I2 to modulator I3. Here input current V is combined with a carrier current C, applied to modulator I3 from carrier source l5; the product 3C+V resulting, among others, from the modulating action of modulator I3. This product, 3C+V, is fed back through band-pass filter I6, which is designed to pass only a narrow band of frequencies including 3C+V, to the input of vario-losser I2, through the variolosser, and thence to the input of modulator I3. We now obtain in the output of modulator I3 a second product 4C'+V.

This second product, 4C+V, is now passed through band-pass filter I1, which is designed to pass only a narrow band of frequencies including 4C+V, and applied to marginal rectiiler 2l.

Marginal rectifier 2l is designed to deliver no output when the input is below a certain fixed value, usually termed the critical value, and to increase its output very rapidly as the input is increased very slightly through the critical value. It may, for example, comprise a vacuum tube detector with the grid biased negatively beyond the cut-off value. (See Vario-losser circuits, by W. R. Bennett and S. Doba, published in Electrical Engineering- Transactions Section, vol. 60, January 1941, page 17.) Fig. la shows a typical output-input curve for a marginal rectifier of this type.

The output of marginal rectifier 2| is applied to the control circuit of vario-losser I2, i. e. the circuit which controls the amount of loss inserted by the vario-losser in currents passing therethrough, and it will be apparent that the output from the vario-losser will be held at a substantially constant value once 4C+V has reached and exceeds the critical value.

Now V, in effect, passes through vario-losser I2 twice, once as the input current over line II and a second time in the product 3C+V. V as appearing in the product G+V has, therefore, traversed vario-losser I2 twice. From all this it follows that for each increase in V at the input of X decibel a loss of decibel will be inserted in input current V as it traverses vario-losser I2 and a like loss of decibel will be inserted in 3C+V as it traverses the vario-losser. The output G+V is therefore subjected to a. loss of decibel for each increase of X decibel in the input V whereby the arrangement functions with the desired action of a 2: 1 compressor.

It will be apparent that vario-losser 'I2 and modulator I3 may be interchanged in position if desired.

assaesa By use of an arrangement as disclosed in Fig. 1,

wherein a single vario-losser fulfills the function of a plurality of such devices, the cost of supplying volume range compression is materially lessened while quality of transmission is not lowered.

Referring now to Fig. 2, an amplifier of the feedback type, constructed for example in accordance with principles set forth in Patent 2,102,671 issued December 21, 1937 to H. S. Black, is schematically represented by box 4I, the ,u or amplification path being designated 42 and the or feedback path being designated 43. A sideband, for example G+V, is applied to modulator 44 and combined therein with carrier current C supplied by source 45. The resulting products include V, the desired frequency and 2C+V, the unwanted frequency. The design of amplifier 4I is assumed to be such that p path 42 will pass the desired frequency V but not the unwanted frequency 2C+V; this latter frequency however is passed by path 43. Band elimination filter 40 serves a part in keeping the unwanted frequency 2C+V out of the /i path.

As indicated by the arrows, the unwanted frequency 2C+V is passed through circuit 43 and band-pass filter 46, which is designed to pass only a narrow band of frequencies including 20+V, and applied to the input of marginal rectifier 41.

Marginal rectifier 41 is of the type described above in reference to Figs. 1 and la, that is it is designed to deliver no output when the input is below the critical value and to increase the output very rapidly as the input is increased very slightly through'the critical value.

The output from marginal rectifier 41 is applied to suitable means associated with path 43 for controlling the loss therein. This control means may, for example, include a thermo-sensitive element as set forth in Patent 2,178,333 issued October 31, 1939 to R. R. Blair.

Once 2C+V has reached the critical value, its amplitude as applied to the input of marginal rectifier 41 is held substantially constant because of the control of the 'resistance in the path by the output from the rectifier. Now if C+V at the input increases I decibel, 2C+V will likewise increase 1 decibel thereby increasing the loss inserted in the path a corresponding amount; this I increase in the loss of the circuit will, in turn, cause a corresponding increase in the gain in the a circuit of the amplifier. In other words for each increase of 1V decibel in the Vinput a like increase in the over-all gain of the amplifier results whereby the action of a 2:1 expander is attained.

Referring now to Fig. 3 wherein the variolosser, marginal rectifier and other elements may be of the same general type described above, the

,-voice frequency input current V and carrier current C1, supplied by source 5 I, are passed through vario-losser 52 and applied to modulator 53. Here they are combined with a second carrier current C2 supplied by source 54. From the resulting products of modulation one, Ci+C2+V is selected by band-pass filter 55, which is designed to pass only a narrow band of frequencies including CiiC'ziV, and applied to the input of marginal rectifier 56. l

The output from marginal rectifier 56 is ap plied to the control circuit of vario-losser 52 and 4 it will be apparent that once CiiCzi-V has reached the critical value, the output from vario losser 52 will be held substantially constant. The product CriCarV varies directly as V and directly as C1. Therefore the produce varies as the sum of the losses introduced for Ci and V. If V at the input be raised 1 decibel, marginal rectifier 56 will attempt to introduce 1 decibel loss in the" product controlling it. To do so, however, it causes only 1/2 decibel loss to be introduced in vario-losser 52 since loss at that point affects both V and C1. Therefore V at the output of vario-losser 52 or CziV at the output of modulator 53 varies at the desired rate for a 2:1 compressor.

Band-pass filter 53, provided in the output of the line, is designed to pass only a narrow band of frequencies which band includes Cz-i-V.

Referring now to Fig. 4, a 2-: 1 ratio compressor is illustrated. Voice frequency current V, impressed on input line 1| and windings 12 of hybrid coil 13, is applied to the input of variolosser 14. 'I'his current after passing through the vario-losser is passed by low-pass filter 15 tothe `output-16. A portion of the voice frequency current V passes over line 11 to modulator wherein it is combined with a carrier current C supplied from source 8'2. 'I'he resulting product CiV is impressed on windings 83 of hybrid coil 13 and applied to the input of vario-losser 14. After leaving vario-losser 14, the current C| V is blocked from output 16 by lowpass filter 15 but is passed by band-pass filter 84 which is designed to pass only a narrow band of frequencies which includes CiV. Ci-V is applied to the input of marginal rectifier 85, the output of which is applied to the control circuit of vario-losser 14. From previous descriptions of marginal rectifier operation, it will be apparent that once the critical value of CiV has been reached, the output from vario-losser 14 will be held substantially constant. q

Now as V, in effect, traverses vario-losser 14 twice, once as applied through hybrid coil windings 12 and again in the product CiV as applied through hybrid coil windings 83, V, as appearing in the product CiV when applied to the control path of vario-losser 14, has traversed the vario-losser twice. For each increase of 1 decibel in V at the input a loss of 1A, decibel will be inserted in V as it traverses vario-losser 14 and of frequencies which includes C. After passing through filter H2, current C is applied to marginal rectier I3, the output of which, once the critical value of C has been reached, acts to hold the output of vario-losser |02 substantially constant.

Now if V, as applied to input |0I, increases by 1 decibel, the loss in vario-losser |03 will increase by 1 decibel. C, which controls vario-losser |02,

in passing through vario-losser |03 is subjected therefore to a loss which is increased by l decibel for each gain in l decibel of V as applied to input IOI. For each increase in loss of '1 decibel in vario-losser |03, therefore, the loss in vario-losser 02 is decreased by 1 decibel whereby a ratio of 2:1 with respect to V in output ||4 as compared to V in input |0| results. The desired action of a 2:1 expander is thereby attained. i

, Band-pass filter ||5 provided in output line I |4 is designed to pass only a narrow band of frequencies which band includes V.

While certain specific embodiments of the invention have been selected for illustration and detailed `description the invention is not, of course, limited in itsapplication to these embodiments. The embodiments described'should be taken as illustrative of the invention andfnot as restrictive thereof.

What is claimed is:

1. In combination a transmission line and a volume range modifier connected in said line for modifying the volume range of currents applied to said line, means for applying voice frea like loss of 1A: decibel will be inserted in CiV as it passes through the vario-losser. That is for each increase in V as applied to input 1| of 1 decibel, the loss applied to V in vario-losser 14 will increase' x/2 decibel whereby the desired action of a 2:1 compressor is attained.

Referring now to Fig. 5, a 2:1 ratio expander using only backward-acting elements is illustrated. Voice frequency current V impressed on input line |0I, divides in the resistance pad, and is applied to No. 1 vario-losser |02 and No. 2 vario-losser |03. The portion of voice frequency current V applied to vario-losser |03 passes therethrough in the direction indicated by the arrow and is applied through line |04 to band-pass filter |05 which is designed to pass only a narrow band of frequencies which includes V. This current is applied to marginal rectifier |00 the output of which, once the critical value of V has been reached, acts to hold the output of vario-losser |03 substantially constant.

Carrier current C, supplied by source |01, passes through vario-losser 03 in the direction indicated by the arrows, through the resistance pad and is applied to vario-losser |02. Carrier current C, after passing through vario-losser |02, passes over line and through band-pass filter ||2 which is designed to pass only a narrow band quency current the volume range of which is to be modified to said line,a path through said modifier for said voice frequency current, means for deriving voice-representing currents from the output side of said range modifier and returning them to the input side thereof to cause them to pass through said range modifier and into said line, control means included in said modifier, means vfor producing a second current distinguishable on a frequency basis from said voice frequency current and having amplitude characteristics of said voice frequency current as modified by passage thrOl-lgh said path, means for rectifying said second current, and a path including a frequency selective device for applying the resulting direct current to said control means of said modifier for controlling the magnitude of change introduced in said voice frequency current by passage Athrough said modifier, said rectifyingl .means being of the marginal type the output from which is substantiallynil while the :input thereto is below a predetermined minimum but rises sharply when said input exceeds said. predetermined minimum amplitude whereby the output from said modifier is held substantially constant, as long as said voice frequency current -exceeds said predetermined minimum, by application of said direct current to said control means. y

2. In combination a transmission line and a volume range modifier connected in said line for modifying, the volume range-of currents applied to said line, means for applying voice frequency current the volume range of which is to be modified to said line, a path through said modifier for said voice frequency current. means for deriving voice-representing currents from the output side of said range ,modifier and returning them to the input side thereof to cause them to pass through 76 said range modifier and into said line, controlI means included n said modifier, a source of carrier frequency current, means for combining said voice frequency current and current from said source' to produce a second current distinguishable on a frequency basis from said voice frequency current andhaving amplitude characteristics of said voice frequency current as modified by passage through said path, means for rectifying said second current, and a path including a frequency selective device for applying the resulting direct current to said control means of said modifier for controlling the magnitude of change introduced in said voice frequency .current by passage through said modifier, said rectifying means being of the marginal type the output from which is substantiall-y nil while the input thereto is below a predetermined minimum but rises sharply when said input exceeds said predetermined minimum amplitude whereby the output from said modifier is held substantially constant, so long as said voice frequency current exceeds said predetermined minimum, by application of said direct current to said control means.

3. In a signal transmission system, the method of compressing the volume range of the signals transmitted thereover which comprises the steps of passing the signal currents through a variolosser device once and controlling the amount of loss introduced by said vario-losser in said signal currents passing therethrough in accordance with currents the amplitude of which has been compressed by passing through said vario-losser twice.

4. A volume range compressor comprising a line section, means for applying voice frequency current the amplitude of which is to be compressed to said line section, a vario-losser connected in said line section and effective when said applied voice frequencycurrentexceeds a predetermined amplitude for compressing the amplitude of currents passed therethrough, 'control means for said vario-losser for controlling the amount of amplitude loss introduced in current passing through said vario-losser, means for effectively passing said voice frequency current -through said vario-losser twice, a source of carrier current, means for combining said voice frequency current as modied by passage through said vario-losser twice with saidv carrier current whereby to produce a control lcurrent distinguishable from said voice frequency current on a frequency basis but having the amplitude characteristics of said voice frequency current as modified by passage through said vario-losser twice, means for rectifying said control current, means for applying the resulting direct current to said control means of the vario-losser, and frequency selective means connected in the output end of said line section, said rectifying means being of the marginal type the output from which is substantially nil While the input thereto is below a predeterf mined minimum but rises sharply when said. input exceeds said predetermined minimum amplitude whereby the output from said vario-losser is held substantially constant, so long as said voice frequency current exceeds said predetermined minimum by application of said direct current to said control means.

5. A volume range compressor comprising a line section, a vario-losser and a modulator connected in series in said line section, a control circuit for said vario-losser, a source of voice frequency current the volume range of which is to be compressed, means forrpassing said voice frefil quency current'through said vario-losser to said modulator, a sourceof carrier current, means for combining said voice frequency current and said carrier current in said modulator, means for selecting one of the products of the combining process in the modulator, means for passingsaid selected product through said vario-losser to said modulator, means for combining said selected product and said carrier current in said modulator, means for selecting one of the products of this second combining process in the modulator, both the first-mentioned and second-mentioned products having amplitude characteristics of said voice frequency current, means for rectifying said second-mentioned product, means for applying the resulting direct current to said control circuit of said vario-losser to control the amount of loss introduced by said vario-losser in currents passing therethrough, and means for selecting and passing to the output of said line section a compressed wave, said rectifying means being of the marginal type the output from which is substantially nil while the input thereto is below a predetermined minimum but rises sharply when said input exceeds said predetermined minimum amplitude whereby the output from said variolosser is held substantially constant, so long as said voice frequency current exceeds said predetermined minimum, by application of said direct current to said control means.

6. A Volume range expander comprising a line section, a wave amplifier connected therein, a

feedback circuit connecting the output of said current in said modulator, one of the products of y said combining process in said modulator being the desired frequency the frequency range of which is to be expanded and another product being an unwanted frequency which is not to be transmitted over the line, said second product having amplitude characteristics of said applied current, means for passing said desired frequency through the amplification path of said amplifier to the output of said line section and for excluding said unwanted frequency therefrom, means for passing said unwanted frequency through said `feedback path and subsequently rectifying the same and applying the resulting direct current to said feedback controlling means and means effective when said applied current exceeds a predetermined minimum for maintaining said unwanted frequency as applied to the rectifying means substantially constant.

7. A volume range expander comprising a ilrst line section, a first vario-losser connected in said rst line section, a second line section connected in shunt with said first line section, a second vario-losser connected in said second line section. a control circuit for each of said vario-lossers, means for applying voice frequency current the volume range of which is to be expanded to said lfirst line section for passage throughsaid first vario-losser, means for diverting a portion of said applied voice frequency currentI to said ysecond line section for passage through said second vario-losser, means for rectifying said portion of voice frequency current after passage through said second Vario-losser and for applying the resulting direct current to the control circuit thereof to control the amount of loss introduced by said second variolosser in currents passing therethrough, means effective when said voice frequency current exceeds a predetermined minimum amplitude for maintaining the output of said second vario-losser substantially constant. a source of carrier current connected to said second line section, means tor passing carrier current and said first vario-losser in series, means for rectiiying said carrier current after passage from said source'through said second vario-losser 1 .ALTON C. DICKIEBON. 

